In this paper, we report on the preparation and magnetic properties of small Pt nanoparticles (2−5 nm size). The control of the preparation conditions allow the physical properties of the nanoparticles to be tuned, giving rise to properties different from those of bulk Pt. We show here the first experimental observation of ferromagnetic behavior for carbon-coated platinum nanoparticles with coercivity up to room temperature (i.e., surprisingly there are no superparamagnetic effects despite the small size of the nanoparticles). Measurements of the surface plasmon resonance showed the presence of itinerant electrons. TEM studies confirmed that the nanoparticles exhibit a large concentration of twin boundaries which reduce the surface energy. The lack of cubic symmetry at the twin boundaries could give rise to permanent magnetic moments. The large spin−orbit coupling of platinum accounts for a huge magnetic anisotropy and therefore for the surprisingly high blocking temperature, above 300 K. The relationship of the magnetic properties with the particle structure is also discussed.